Preparation of cadmium sulfo-selenide reactants



i atented Feb. 7, 1 950 PREPARATION OF CADMIUM SULFO- SELENIDE REACTANTS Guy C. Marcot, Lynchiiurg; Va., assignor to American Cyanamid Company,

New York,

N. Y., a corporation of Maine No Drawing. Application June 28, 1946, Serial No. 680,098

3 Claims. (01. 106401) This invention relates to the production of cadmium red pigments and is concerned in particular with a novel method of making both old and new products.

' 'As" used in the present invention, the term cadmium red pigments is used to designate any of a variety of calcined products ranging in color from bright orange to a dark maroon and containing both CdS and CdSe. These components appear to form a solid solution in the finished pigment, constituting the principal colored constituent thereof. Control of the color shade is obtained by varying the relative proportions of CdS and CdSe in the solid solution, approxi- While these proportions are known, various shades having been previously obtained by a number of proposed methods, these methods have not been wholly satisfactory and are subject to a number of procedural and economic disadvantages. Particularly is this true as to the production of the darker shades.

In addition to preparing these pigments as pure types, i. e., forms in which the solid solution is the principal constituent, they are also commonly prepared as lithopone or extended" types. This is usually done by including a suitable amount of an inert extender such, for example, as BaSOl, in the crude before calcination. These extenders may be incorporated in the crude in any desired manner, usually, however, by coprecipitation with one of the components thereof. vention is readily adapted to the production of pigments of any of these types and its advantages .apply equally in the manufacture of either pure or extended forms.

In the past, cadmium red pigments, such as those with which the present invention is concerned, have been obtained by combining a series of procedural steps, each in turn selected from various proposals, to produce a calcinable crude.

Such procedures may be roughly divided into two i general classifications; precipitation of the crude fcompounds of indefinite structure, usually designated as cadmium sulfo-selenides; and preparation of the components as a mechanical mixture.

The process of the present inl a mental Se.

' tiliz ed Se.

The sulfo-selenide type of crude is usually obtalned by dissolving selenium in a sodium or barium sulfide liquor and carrying out a strike of this liquor and a solution of a soluble cadmium salt. The resultant cadmium-bearing precipitate is used as the crude. In preparing the mixture type of crude, CdS is separately prepared and then combined, usually in a slurry, with a CdO reactant, which may be CdO per se or a compound reducible during calcination to CdO; and with an Se reactant, preferably ele- In either type of preparation, some sulfur is also usually present in the crude.

In either case, the crude to be calcined, however prepared, is collected, dried if necessary, and finally calcined. At suitable temperatures, this results in conversion of at least part of the selenium and an equivalent amount of cadmium compound or compounds to CdSe. This reaction is accompanied by the concurrent liberation of sulfur dioxide and also, in previouslyused processes, some free sulfur which is volatile and eliminated at the usual calcination temperatures of about 1000-1400 F.

However, as noted above, the various procedures used in the past to prepare cadmium red pigments are all subject to certain difiiculties. Perhaps the most common of these stem from the problem of replacing the sulfur in cadmium sulfide by the less active selenium. Much Work has been done on this problem and the resulting proposals of methods for forcing the reac tion in the desired direction have been highly varied. They range from the obvious and simple expedient of providing a large excess of the Se reactant or reactants to the complicated process of calcining in an atmosphere of vola- None however have proved wholly satisfactory.

So troublesome is the sulfur replacement reaction that in the past production of deeper shades than about medium to dark red was con-, sidered commercially impractical. The previously-used processes all are subject to objection in that regardless of the shade being produced they result in an excessive loss of Se, a critical economic factor. Further, they result in unreliable production, only with extreme difficulty, of the dark red through the maroon and deep maroon shades.

It is, therefore, the principal object of the present invention to develop a simple, reliablycontrollable procedure of making cadmium red pigments in which the amount of available Se in the crude is more fully recovered, i. e., converted to CdSe, than was possible with previously used procedures. Such a process, in addition to its improved economy in production, should also make possible simple and reliable formation of those darker shades. necessitating the higher CdSe contents.

In general, accomplishment of these desired objects is simply and readily obtained by compounding the Se and CdO ingredients in the crude in the novel molar proportions of 3:22. In

addition, free sulfur is neither included in the crude nor allowed to formduring calcination.

Se and CdO in controlled proportions of 3:2. A consideration of the SezCdO ratio as a control factor is not wholly new with the present inven-- tion. Perhaps, the best of the previously-developed considerations in this respect are presented in. U. S. Patent No. 2,134,055 to W. F. Meister, wherein. the optimum conditions are considered to be. found when the SezCdO ratio is 2:1. The. 3:2 ratio of thepresent invention has been considered to be. within the range of proportions which, are eitherimpractical or impossible to use. It. is quite. surprising, therefore, that, the novel proportions of the present process are. so successful in practice.

A secondary, but. farfrom negligible, factor in obtaining the, desirable results of the present invention lies inl the novel procedure of eliminating as far aspossible the presence of free sulfur, either in the crude before calcination or liberated during calcination. In the previously used processes varying quantities of free sulfur have been presentv during calcination. Opinions as to its presence have varied from its being a necessary but harmless diluent to its being both desirable and advantageous. In some cases it has, been deliberately added. Consequently it is quite. surprising that in the present invention it has been found that free. sulfur present during calcination is not only unnecessary but actually deleterious. Its presenceresults not only in the production of inferior pigments but interferes to a large extent with the "effective conversion of the available Se to CdSe.

As noted above, a number of procedures for producing the mixture of ingredients that consti' tutesthe crude are known. It is an advantage "of the process of the presentinvention that any of them may be used if so desired. However, care must be taken, whatever the process that is used, that it be controlled to produce the desired Se-ICllG-ratioand preferably also that it be controlled to eliminate free sulfur;

'Inorder to insure this control it was found in developing the present invention that a process in which the desired'components are individually-prepared and'then mechanically admixed oncombined before calcination is to be preferred. Thispracticeappears toallowsimpler and more reliable control of the formulation. Accordingly, while the present invention is not necessarily limited thereto, it will be discussed as illustrated by this procedure.

The essential components of a mixture type of crude are the Cdsreactant; the CdO reactant, preferably in the form of CdCOa; and the Se reactant; preferably as elemental selenium. The preparation and use of these reactants may be illustrated; in the following manner:

THE CdS REACTANT A CdS reactant, containing the extender mate'- rial, is usually prepared by a strike of a solution of a soluble. cadmium salt, preferably CdSOd, with a BaS liquor. This-results in the precipita- 4 tion ofCdS, and-wherefidsm is used, the comecipitation. of an equivalent amount?- of Bil-S04. Sufficient BaS should be used to insure complete precipitation of the cadmium. Subsequently, the total amount of extender may be adjusted as desired by the addition of the requisite amount of separately prepared 32504. In preparing a pure type, the Basjmay be replaced by an alkali metal sulfide to form a soluble sulfate which is washed out.

Although not a part of the instant invention,

it usually is desirable that a small quantity of an anti-iiuxing agent be present in the crude, as is set forth more fully in my copending application, Serial No. 680,100, filed of even date. This: agent may be. conveniently added either before. or after adjusting the extender content. The anti-fluxing agent. is preferably; but. not necessarily, an insoluble orthophosphate such as that of. barium. or aluminum. It may be formed readily in. situ by adding H3PO4, which is converted; to the phosphate by barium ions already; present in solution or by a reactant such as aluminum sulfate, specifically added for the purpose. Use ofr0.2-0.5% is a good general procedure. The solids. content. Off the. slurry, which may be washed it so desired, for the.- purposes of the present process, is not collected.

THE CdO REACTANT As noted above the CdO content of the crude either may be added perse or as a compound readily converted to CdO during calcination. Cadmium oxide itself may be quite readily prepared, for example by reacting a solution of a cadmium salt, usually the sulfate, with about two mol equivalents of sodium hydroxide, collecting, washing and, if necessary, drying the precipitate. Actual practice,however, shows that ading the CdO reactant. as a material such as CdCOs, which is easily converted to CdO. during calcination, is preferable. The material so used is herein referred to as the CdO reactant regardless of itsactual composition.

ACdCOzslurry'may be prepared by a strike of a solution'ofa cadmiumsalt, such as the sulfate, with a solution containing about a molecular equivalent of an alkali carbonate or about two mols of an alkali bicarbonate. In accordance with the present invention it has been found that the latter produces a CdCOx which when used in the composite results in a more desirable final pigment. A slight excess; usually about 0.1 mol per mol ofv bicarbonate, is helpful but not essential. A batch strike byeither' adding the bicarbonate to the cadmiumisalt; or by adding the cadmium salt to the bicarbonate is definitely preferable over asimultaneous. strike preparation. The simultaneous strike does not produce as crystal!- line a product as the batch strike. Further, the simultaneous strike appears to give a somewhat larger content of combined sulfate, probably present as a basic sulfate of'cadmium.

Because oftheir fiuxing effect, the presence of excessive amounts of. soluble salts. in the crude may produceanadverse effect during calcination.

Therefore thev cadmium carbonate. precipitate sh'ould'be Washed, preferably as much as the dis.- persive. nature. of. the precipitate will allow, to reduce this salt content. A good practice is to continue washing until the residual soluble salt content is reduced to about-0.5% orless ofthe initial content. Washing. below this point is usually. physically impractical, at least when washing by the decantation procedure.

shown in 'my copending application, Serial No.

680,101, filed of even date, it is preferable't'o form a separate slurry-by using a suitable 'wet-. ting agent. 'This'ma'y' be any of several materials. However, since organic matter and/or soluble alkali metal salts are undesirable in the crude,

the preferred wetting agent is BaS. To prevent 1 sulfur formation during calcination, an excess of the latter should be avoided. In general practice, slurrying the powdered selenium in about a 0.5% aqueous solution of BaS produces good results.

FO RMULATING THE CRUDE v In preparing the composite, it is perhaps preferable that the Se slurry beadded to the CdS slurry and then the CdCO3 slurry added thereto. However, the exact sequence may be varied if so desired. The combined slurries are thoroughly agitated to produce as intimate an admixture as possible. Subsequently the solids content of the mixed slurries is dewatered, washed if so desired, and dried. This driedproduct constitutes the calcinable crude.

Some of the entrained or entrapped soluble salts in the CdCOs slurry will be present in thecombined slurry, As shown in my previouslyidentified copending application, a part of this salts content is released when the slurries are admixed. The released salts may and should be washed out. The remaining soluble salts content may be made to serve as a fluxing material during calcination.

However, the residual amount of soluble salts in the mixed slurries isindefinite. Usually it exceeds that desired for fluxing p poses. In any case, their fiuxing, effect cannot be too. reliably predicted. It is for this reason that, as was noted above, an anti-fluxing agent is prefer,- ably added to the CdS slurry during preparation thereof. Where this has been done, the resultant crude normally will be deficient in fluxing materials. The latter can be added in controlled amounts to the crude before drying and calcination, to give a predetermined optimum fiuxing' effect. Any' soluble compatible salts such as sodium and/or potassium sulfate, may be used. Amounts required will range from about 0.15-- 0.35% of the solids weight. Use of about0.3%' is a good general practice.

CALCINATION will vary somewhat in accordance with the actual proportions of materials used and the presence or absence of fiuxing agents. In general I have found that temperatures ranging from about 1000-1400 F. are suitable, calcination being} continued for sufiicient time to insure substan tially complete reaction. It should be noted that these-"temperatures are those of the pigments themselves during calcination. When using a mufiie furnace, for example, the gas temperatures may be some 200-400 F. higher.

' The invention will be described and illustrated more fully in conjunction with the following examples in each of which the various reactants are prepared and treated in accordance with the foregoing discussion. The examples, however, are varied asto the SezCdO ratios in the crude in order to show the efiect thereof both on the color or mass tone values and on the efficiency of conversion of the available Se. Where in these examplesa CdSO4 liquor is-referred to, a 36 B. solution is used andthe BaS liquor is about a 1'7? B. solution. Except as noted, all parts are by weight.

' EXAMPLE :1

Se:CdO ratio'oj 4:1

-A CdS reactant was prepared by reacting 59.7 parts ofCdSO4 liquor and 50 parts of BaS liquor. Finally, about 0.22 part of H3PO4 and a suflicient quantity of BaSOaabout 24 parts, to produce a total content of about 91 parts of blanc fixe were added. For. the .CdO reactant 10.8 parts of C6804 was added batch-wise to about 9.1 parts of a 10% aqueous solution of NaHCOa, this being equivalent to about 2.1 mols of the NaHCOa per mol of 'CdSO4. A Se slurry was prepared by pulping 16.5 parts of Se powder to about 50% solids with a 0.5% BaS solution. The Se slurry was added to the CdS slurry and finally the CdCoa slurry added thereto. The whole was thoroughly admixed, dewatered, dried and calcined for about 10 minutes over a temperature ranging from an initial 1150 F. to a final 1350 F. The resulting pigment was found to possess a mass tone of excellent quality and a depth which was substantially 'a color match fora standard dark red cadmium; red pigment.

From the foregoing it is apparent that the CdOzSe ratio of 1:4 was adjusted so that the production of CdSe would appear to follow the reaction:

EXAMPLE 2 Se:C'dO ratio of 2:1

. Example 1 was repeated with the exception that 21.6 parts of CdSO4 liquor and 18.2 parts of NaHCOa as a 10% solution were used in preparing the CdCOa slurry. The final calcined pigment possessed a mass tone of excellent quality of a dark red shade.

The proportions are calculated to give a CdzSe weight ratio in the final pigment of 37.7:62.3 wherein the CdSe formation would follow the reaction 7 These are the proportions indicated as'optinmm in U.:S. Patent 2,134,055. :Analysisof the oagloined pigment, howevenindicated a Se :d ratio-of:0.324 whereas the ratioas formulated was :377. Ac-

cordingly the Se conversion to GdSewas only In order to illustrate the prererred eacnse ratio of the-present inventionfExain-ple 1- was repeated usiiig th idlltjwiiig ingredients: (-di) fillthe C018 lithODOrie preparation- 5213 p'arts 6f CdSOs liquor, 44 parts of has 1iqiiorand--32 BaSOi; '(b) for the CdCUs preparation-29:0 parts of CdSOr'liquor and'2'4B-p'arts of'NaHCQa as a aqueous solution; and (*c") for the Si: s1u1ry16.5 parts of Se powder. Calcina-tion'wae carried out for about 12 minutes from an initial temperature of 1150 F560 "af'fi'nal 1350 F. The calcined pigment possessed aii'ias's tone of excellent quality, in depth a deep maroon shade.

It will be seen that'the proportions tisedih this test are intended to produce asetcdweiight ratio in the final product of 37.7:623'and warms-Case formation will followther'eaotion 2CdO+CdS+3Se-'- 3CdSe +SO2 from which equation 'it W'iilfbe seen thatino nee sulfur is present or formed during caloihatioh and that the Volume; of gas prouuceafperunit of CdSe formed is 'niiiiiiniied. Analysis of the pigment indicate'da SetCd r'atio 6110.365 as b'iih'ipared with the .formii'l'aitdratio of 0.377.

corresponds t the hig'hlysatisfao'toiwSetdifiirsion of about 97%. U I

From the foregoing example "it will be "seen that an excess of Sein theorude'is'not deleter ous so far as shade andinass't'one i eoncemeo The principal defect in using an SeiCdO'ratio'Igreatei than 3 :2'isanecon'oin'io one. The depth dresses produced is not consistentwithfthe s eniumeensumptionl With the correct ratio of '3 mesmersion of Se to Cdse is almost complete.

EXAMPLE 4' SezCdO ratio of 4:3

In order to show the effect of "stinrurtne'r decreasing the 'Se :CdO ratio, i.-e., a departurefrom the optimum in using an excess of 'the-CdO'reactant, Example 1 was repeated usingthe follow ing ingredients; (:1) for the CdS lithopone prepaf ration CdsOi, 48;!) parts as a-36 Be. -liquor, -(-b) B as-41 parts as a 17 B. liquor, and (o) BaSO4sufiicient to give a total of -9-1 i-'-pai*ts- (36 parts added), ((2) in the GdCGapreIiatation CdSO4, 32.4 parts as'a 36B. 1iqI10r'8nd-'N8,-HC03,- 27.8 parts as a 1D%-s'olution,:-(e.) intheSeshirr-y preparation-Se powder 16.5 par-ts; sulfur parts. The dried crude was calcined over 10 minutes from an initial temperature of 1100 F. to a final temperature ofl-300" F. Thefinal pigment possessed a mass tone with a depth of a light maroon shade but the qaulitywasohl'y' fairdue to'a slight brown cast. p

The ingredient proportions are ealoilfa'ted th give a 'SezCd Weight ratioof 37 7562. 3 in tl'iefinit'i product wherein the odes-immense neura -rm low the following reaction:

SefCdG "rdtiob) '1 :21

In order to further illustrate the efi'eotof a still greater divergence from the optimum in the amount of excess (-JdO in the crude, Example -1 was repeated using the --following ingredients: (-a'). in the -Gd$ lithopone preparation -Cdsos, 38; parts as a 36 B.liquor -BaS,--32 parts as a 17 B. liquor; BaSOFATparts to give a total'of 91- parts; (12) in the -CdCOs preparationcdSOn parts asa 36 'B'. liquor; Na/H003, 36.4 parts -as a10%'solution (c) in =the-Se slurry prepare ti0n-Se;po-wder, 1-6.4 vparts; S, 3.4 parts. The dried -orude was calcined for 10 minutes from an initial temperature of ll-00 F. to -a-final temperature of l 300-F.

Ingredient proportions were =caleul ated to give a Se :Cd Weig-ht r-atioof 37.72623 in the finaLpr-oduct, wherein the CdSe formation would follow the reaction '2'S+4CdO+4Se' 4CdSe i-2SO2 Analysis 'of the product exhibited a Se 26d ratio of 01347 rather thanthe formulated 0:377. This corresponds to a Se conversionof 92% which is 'good. The pigment quality, however, was poor, possessing a brownish tinge and bein'g defioient tostandard.

From the foregoing examples it will be seenthat the presence of an excessive amount of CdO in the crude hasa somewhat different effect'than ex'dess Se. Unlike the latter-it does not produce ain ir oe's'sii e lossdf Se. l-t doesfihowever, result ni -pigments of poor quality mass-tone and usually deficient in tint to'ne.

'SetC'dUr'dtib of 1:1 with ezrce'ss S in diner toshow the deleterious 'fi'eot of tree sulfur, Example 5 was repeated except that prior to calein'ation "approximatel '100 parts of the dried erudewas dr yblerided with '2 parts of sur- -'I his 'quami-ty ofexcess sulfur is equivalent to "-an ingredients formulation for CdrS'e forma aco'ordi'ng'to the-following The final'pigment possessed a mass tonewith'a depth equivalent only to a dark re shade rather than the "deep maroon which'shouldbe produced by eflioient'oonversion-of the aniount'of Se used;

In addition the quality Was very 15001 due to '9. roiioun'eeduark'browncast.

' The effect of the excess sulfur inclusion prior to calcination appears to be to lighten the mass tone without improving the quality, further in- =m dioating. that the quality obtainable is largely .0377, corresponding, therefore, "to a Go 'c'o'nversion of onl'y 77%. The presence of excess sulfur is therefore, as indicated by the prior examples, shown to be a very major factor in determining the extent of the Se recovery.

While theforegoing examples-have been concerned with the production of lithopone or extended types of pigments, asnoted above {the invention is ndtlimited thereto. Its application to the production of pure -type-pigmentsis shown .inthe following example.

9 EXAMPLE '1 Pure type pigment-Maroon shade 279 parts of CdS, prepared by a simultaneous strike of 2.35 molar CdSO4 liquor and 1.0 molar NazS liquor and washing the resultant precipitate was slurried in 2000 parts of water and 2.8 parts of H3PO4 and an equivalent amount of A12(SO4)3 was added thereto. 177 parts of CdCOs, prepared by a strike of 1.76 molar CdSO4 liquor with 1.2 molar solution of NaI-ICOa was slurried in 1200 parts of water. 123 parts of Se powder was made up into an approximately 50% pulp in 05% aqueous BaS liquor. These slurries were combined and adjusted to a total volume of about 3500 parts. A sample of the solids content indicated the soluble salts content of about 0.2%. This was adjusted by the addition of aqueous NazSO; to about 1.0% and the slurry dewatered and dried. The dried crude on calcination over minutes from an initial 1200 F. to a final 1350 F. produced a pigment of excellent mass tone and a depth of shade equivalent to a maroon standard.

The effectiveness of the practice of the present invention in utilizing the Se content of the crude may be clearly summarized by a comparison of the Se consumption using the SezCdO ratio of 2:1, which is the best practice of the previously-used processes, as set forth in U. S. Patent 2,134,055 with the Se consumption in producing the same color shades using the SezCdO ratio of 3:2 of the present invention. This comparison is set forth in the following table:

1 M01 ratio of Cd0: Se used in formulating crude.

The savings become more pronounced in proadvantage of the present invention in addition to the fact that the darker shades can be readily and reliably produced.

I claim:

1. In making cadmium red pigments, the method of improving the selenium conversion and color values which comprises, forming a calcinable composite comprising an admixture of cadmium sulfide, metallic selenium and a cadmium oxide reactant, said reactant being selected from the group consisting of cadmium oxide and compounds yielding cadmium oxide under calcination conditions, said composite being free from uncombined sulfur and having a mol ratio of metallic seleniumzcadmium oxide of 3:2 and a cadmium sulfidezmetallic selenium ratio of at least 1:3 whereby liberation of free sulfur during calcination is prevented, and calcining said composite.

2. In making cadmium red pigments, the method of improving the selenium conversion and color values which comprises, forming a calcinable composite comprising an admixture of cadmium sulfide, metallic selenium and cadmium carbonate, said composite being free from uncombined sulfur and having a mol ratio of metallic seleniumzcadmium oxide of 3:2 and a cadmium sulfidezmetallic selenium ratio of and at least 1:3 whereby liberation of free sulfur during calcination is prevented, and calcining the composite.

3. A process according to claim 2 in which the cadmium carbonate is prepared by a batch strike of a cadmium sulfate solution and a solution of an alkali metal bicarbonate.

GUY C. MARCOT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,115,080 O'Brien Apr. 26, 1938 2,248,408 Juredine July 8, 1941 

1. IN MAKING CADMIUM RED PIGMENTS, THE METHOD OF IMPROVING THE SELENIUM CONVERSION AND COLOR VALUES WHICH COMPRISES, FORMING A CALINABLE COMPOSITE COMPRISING AN ADMIXTURE OF CADMIUM SULFIDE, METALLIC SELENIUM AND A CADMIUM OXIDE REACTANT, SAID REACTANT BEING SELECTED FROM THE GROUP CONSISTING OF CADMIUM OXIDE AND COMPOUNDS YIELDING CADMIUM OXIDE UNDER CALCINATION CONDITIONS, SAID COMPOSITE BEING FREE FROM UNCOMBINED SULFUR AND HAVING A MOL RATIO OF METALLIC SELENIUM: CADMIUM OXIDE OF 3:2 AND A CADMIUM SULFIDE: METALIC SELENIUM RATIO OF AT LEAST 1:3 WHEREBY LIBERATION OF FREE SULFUR DURING CALCINATION IS PREVENTED, AND CALCINING SAID COMPOSITE. 